Desaturation is a necessary step in the modification of dietary linoleic and linolenic acids to arachidonic and docosahexaenoic acid, respectively. The levels of these acids are lowered by ethanol in the liver and brain and it is likely that this leads to some of the pathological consequences associated with alcoholism. We are therefore examining the basic biochemistry of the desaturase system and observing the effects of both diet and alcohol exposure on enzymatic activity. The initial phase of this work requires the development of methodology for assay of desaturase activity both in vivo and in vitro using stable isotopically labeled fatty acids by selected ion monitoring electron capture negative ionization GC/MS. The characterization of the chromatographic, electron impact , and negative ion electron capture mass spectra of the pentafluorobenzyl (PFB) derivatives of the various fatty acids has been initiated. Methods for producing the PFB derivatives from polar and neutral lipids as well as subsequent purification procedures and recovery experiments are under development. In vitro and in vivo studies with rats, mice,and guinea pigs will provide a basis for the development of human protocols. We investigated the effects of both lipid dietary modifications and chronic ethanol exposure on the production and release of 2-series prostaglandins from rat brain slices. Rats were given diets with different fat sources for two weeks and exposed to alcohol via inhalation for an additional week. Results of the in vitro procedure showed that both diet and ethanol treatments significantly affected prostaglandin release from brain slices.